[GreenKeys] Current loop not providing (enough) current

[Zach Zarzycki]

Here is the 2 points I'm hooking up at - would be pins 15 and 26 (also my
test points that show the 48 ohm)
https://imgur.com/a/TGrywcg
https://imgur.com/FQKJe3C


On Mon, Sep 23, 2019 at 7:55 PM Zach Zarzycki <shardsofaperture at gmail.com>
wrote:

> I'm thinking I need to boost the voltage overall. The darn loop only shows
> 48 ohms which feels wrong when I grab each end of the loop with the meter.
> The schematics are cryptic as all get out but I'm positive on this, where
> it shows the plug for the table, is where I need to wire my loop in.
>
> I'll have to snap a photo of the schematic I'm looking at, but seeing as
> how they keyboard works to TX to the computer terminal, I'm pretty sure I'm
> where I should be.
>
> I went on mouser today and bought the linear 120vdc supply so when that
> comes in I'll try hooking that up though a 2k and see if it makes a
> difference.
>
> I will say there are 2 5k resistors on the unit in different places, but
> they're not in the loop so to speak, especially as seeing I only read the
> 48 at the +/- twx line connection (where I hook my loop power supply and
> board into).
>
>
> *From:* js at teletype.net
> *Sent:* 22 September 2019 12:26
> *To:* greenkeys at mailman.qth.net
> *Subject:* Re: [GreenKeys] Current loop not providing (enough) current
>
> I'm certain that's the problem. I've done some testing on my Models 15 and
> 19 in the past, and if the 26 magnets are in the same realm of tolerances,
> they'll start to become useless at around 50mA, give or take. 25mA is much
> too low a current to have any effect.
>
> The keyboard is keying the loop successfully since that's just mechanical,
> and the board can pick up signals even lower than 20mA, but the selector
> magnets can't.
>
> The question is why. Looks like you expected 96 volts and so calculated
> 1600 ohms resistance to get 60mA. However, it sounds like there's more
> resistance in your loop. A few hundred ohms is expected, but you've got an
> additional 2.2k ohm and change.
>
> I'm willing to bet the loop may already have a 2k ohm resistor in series
> somewhere-- maybe installed within the machine itself if you have nothing
> else on the loop. Try measuring resistance in the full loop with and
> without your extra resistors, and I'd guess you'll see the same thing. As
> long as you see roughly 2000 ohms on the loop without your extra resistors,
> it will be safe to power it without the extra resistors. At 96 volts,
> however, you'll get a current of only around 40 - 45mA, which probably
> won't reliably trigger the magnets. But who knows, maybe it'll work fine--
> it's always worth trying.
>
> If not, you'll either need to locate and replace that 2k resistor with
> your 1600, or you'll need to boost the voltage.
>
> -
> Jordan Spencer Cunningham
> teletype.net
>
>
> On Sat, Sep 21, 2019, at 9:37 AM, Zach Zarzycki wrote:
>
> Well - I have my 26 hooked up though the volpe board and have managed to
> get as far as when I type on the teletype's keys the terminal window on the
> laptop accurately displays what I've punched up.
>
> However, typing on the teletype doesn't cause the keys to punch paper, and
> typing on the laptop keyboard won't cause the teletype to do anything.
>
> I'm set up with a boost converter and a 12v 3amp power supply that SHOULD
> be supplying roughly 60mA at 97 volts though the loop after being boosted
> and then ran though about 1.6k wire wound resistor (600 ohm and a 1kohm in
> series, both 25W rated).
>
> The problem is when I read the voltage out of the booster, it shows  97v,
> but the most I've seen any amperage on any test point is 25ma, a far cry
> from 60ma. I'm wondering this this is possibly the reason for the computer
> keyboard not being able cause the keyboard close the magnet and select a
> letter then type.
>
>
> My gut says maybe? But I find it very odd though that I can make the
> greenkeys act as a keyboard, so maybe I'm missing something else.
>
> Zach
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